I read once that early Ford boxes used something like 600 wt oil, but try to find that stuff anywhere. So I just fill the 56 Ford pickup boxes I have with 85-140 wt rear end lube. It might not be the right stuff, but it has been in my 27 for over 20 years and seems to be lubing it ok.

I considered putting a grease gun in there and filling it with grease, but haven't seen the need to do that yet.

Don